1. Field of the Invention
This invention relates to a pattern inspection apparatus for inspecting a to-be-inspected object and more particularly to a pattern inspection apparatus which inspects an original such as a reticle or mask used for transferring a fine circuit pattern onto a photosensitive substrate such as a wafer or glass plate or inspects the pattern defect of a substrate itself such as a wafer or glass plate on which the pattern on the original such as a reticle or mask is transferred.
2. Description of the Related Art
In the conventional pattern defect inspection apparatus, a to-be-inspected object such as a reticle or mask is illuminated in a spot form or rectangular form by guiding continuous light generated from a light source such as a lamp or continuous wave (CW) laser to the to-be-inspected object via an illumination optics. Light from the illuminated area is collected by means of an imaging optics and an image of the illuminated area of the to-be-inspected object is formed on the light receiving surface of an opto-electronic detector such as a line sensor or an area sensor using a CCD (Charge Coupled Device). An opto-electronic detection signal (image information) from the opto-electronic detector is input to an image processing apparatus which in turn performs the image processing operation for detecting defects of the to-be-inspected object.
At this time, the image processing apparatus can acquire a two-dimensional image of the to-be-inspected object by two-dimensionally moving a moving stage having a to-be-inspected object placed thereon in synchronism with fetching of image information (fetching of an opto-electronic detection signal from the opto-electronic detector) to perform the image processing operation. In this case, reference data (data formed at the time of designing of the to-be-inspected object) used for determining whether the to-be-inspected object is good or not is stored in a memory section included with the image processing apparatus. Therefore, it becomes possible to detect errors such as damage, defects, dust and foreign matter on the to-be-inspected object by comparing image information from the opto-electronic detector with the reference data by use of a comparing/determining section included with the image processing apparatus.
Recently, circuit patterns of semiconductor manufacturing masks are made finer year by year and the resolution required for a defect inspection apparatus becomes severer. In order to enhance the resolution, methods for making the wavelength of light from a light source shorter and attaining high NA of an objective lens are provided, but the method for attaining high NA is not preferable since the depth of focus is made smaller. Further, the wavelength of a light source of a stepper for transferring a pattern onto a semiconductor substrate by use of a mask is made shorter year by year, and at this time, it is preferable that the wavelength of a light source of the inspection apparatus is made substantially equivalent to that of the stepper in order to inspect the phase shift mask well. For the reasons described above, it is indispensable to make shorter the wavelength of light from the light source used in the defect inspection apparatus.
As a light source used in the stepper, a KrF excimer laser with the wavelength 248 nm is already used and a light source using an ArF excimer laser with the wavelength 193 nm is in the development stage. Therefore, it is expected that a defect inspection apparatus using the light source with the wavelength of approximately 193 nm will be realized.
At present, as a light source having a sufficient intensity at the wavelength of approximately 193 nm, an excimer laser and excimer lamp are known, but it is difficult to practically use a lamp having large line width in a deep-UV region in which achromatization cannot be attained. Therefore, it becomes necessary to use an excimer laser, but it is difficult for the excimer laser to continuously emit light in principle and such an excimer laser is not realized at present. As a result, a pulse light source is used as the light source, and in this case, the repetition frequency of the pulse limits the sensing frequency.
For example, since the repetition frequency of the KrF excimer laser which has been already practically used is 1 to 2 kHz, the performance of a line sensor which can effect the sensing operation at 30 kHz cannot be fully exhibited even if it is used. Since the repetition frequency of the laser is limited, the number of pixels which can be fetched for each pulse must be increased in order to enhance the throughput.
Therefore, it is necessary to increase the number of pixels of the line sensor, but a limitation is put on increasing the number of pixels of one sensor, and therefore, it is considered to arrange a plurality of sensors. First, a method for arranging line sensors in parallel at preset intervals is considered, but since it is necessary to continuously move a sensing portion by a distance corresponding to the interval between the sensors, jump the sensing portion to a portion which is not yet scanned and repeatedly perform the above operations, the apparatus is made complicated and difficult to be realized.
Secondly, a method for arranging a plurality of line sensors in a line in the arrangement direction of the pixels is considered, but in this method, it becomes necessary to enlarge the visual field of the objective lens since the total length of the sensor is enlarged and it becomes difficult to eliminate the aberration of the lens. Thus, the line sensor cannot be suitably used for increasing the number of pixels.
On the other hand, it is considered to use an area sensor in the inspection apparatus using the pulse laser light source, but concrete methods are not studied so far and are not yet put into practice. Particularly, when the area sensor is used, it is considered that movement of the to-be-inspected object and sensing timing become important, but these points are not studied at all in the past.
Thus, there is a strong possibility that a pulse laser is used in the inspection apparatus in order to enhance the resolution. However, if the same sensing method as that used when the continuous light source is used as the light source is used as in the conventional case, the throughput is extremely lowered. Therefore, it is strongly required to realize a sensing method which is suitably designed for the pulse laser.
There is a need for solving the above problem and providing a pattern inspection apparatus and pattern inspection method capable of efficiently inspecting the entire surface of a to-be-inspected object by using a pulse laser and area sensor.